1. Field of the Invention
The present invention relates to a spindle device provided in a machine tool or the like and including a cooling flow passage formed inside the spindle device to allow a cooling medium such as a coolant to flow.
2. Description of Related Art
In spindle devices, in general, heat may be produced in bearings for a spindle, a motor that rotates the spindle, and so forth, and the spindle may be heated under the influence of heat produced at a machining point. The produced heat, if no measures are taken against it, may cause thermal expansion of the spindle to deteriorate the machining accuracy, or may break the bearings. Thus, some spindle devices according to the related art are provided with a structure for cooling a spindle. In a spindle device disclosed in Japanese Patent Application Publication No. JP H06-31585 A, for example, a cooling flow passage in the shape of a spiral groove is provided inside the spindle device, and a cooling medium such as a coolant is caused to flow in the cooling flow passage to uniformly cool the entire spindle device.
It is true that the cooling flow passage formed in the shape of a spiral groove in the spindle device disclosed in No. JP H06-31585 A allows the cooling medium to flow at an increased flow rate in the cooling flow passage to improve the cooling efficiency. If a compressible fluid such as air is used as the cooling medium, or if an incompressible fluid such as a coolant is used as the cooling medium but the cooling flow passage is not completely filled with the cooling medium (that is, air or the like is mixed in the cooling flow passage), however, a fluid pressure may be produced by the axial-flow pumping action during acceleration and deceleration. Thus, if the fluid pressure is increased, the cooling medium may leak at a rotary joint portion, for example. Further, if a large amount of the cooling medium leaks at the rotary joint portion, or if a high fluid pressure is applied in the direction of causing a reverse flow of the cooling medium, a sufficient amount of the cooling medium may not be supplied into the spindle device.